[MODEL] Call Kaminari::Hooks.init only when available

See: kaminari/kaminari@653143b

Originally reported by @twe4ked in elastic#660.

Closes elastic#660

Author: karmi

elasticsearch-model/README.md

@@ -303,7 +303,7 @@ In a Rails controller, use the the `params[:page]` parameter to paginate through
 To initialize and include the Kaminari pagination support manually:
 
 ```ruby
-Kaminari::Hooks.init
+Kaminari::Hooks.init if defined?(Kaminari::Hooks)
 Elasticsearch::Model::Response::Response.__send__ :include, Elasticsearch::Model::Response::Pagination::Kaminari
 ```
 

elasticsearch-model/examples/activerecord_article.rb

@@ -26,7 +26,7 @@
   end
 end
 
-Kaminari::Hooks.init
+Kaminari::Hooks.init if defined?(Kaminari::Hooks) if defined?(Kaminari::Hooks)
 
 class Article < ActiveRecord::Base
 end

elasticsearch-model/test/integration/active_record_pagination_test.rb

@@ -30,7 +30,7 @@ class ::ArticleForPagination < ActiveRecord::Base
             end
           end
 
-          Kaminari::Hooks.init
+          Kaminari::Hooks.init if defined?(Kaminari::Hooks)
 
           ArticleForPagination.delete_all
           ArticleForPagination.__elasticsearch__.create_index! force: true